When a fault occurs in a power transmission network, travelling waves arise which move along the line. It is known to use the direction of movement of these travelling waves at a measuring point to determine the direction to the location of the fault. U.S. Pat. No. 3,956,671 (Nimmersjo) discloses how to utilize the fact that in those travelling waves, which from a fault point move in towards the measuring point of the protection device, the current and voltage waves have different signs. If the voltage is designated u, the current i and the wave impedance Z.sub.0, the equation u=-Z.sub.0 i is obtained in the case of an internal fault and a fault lying ahead, respectively. If the fault is located behind the measuring point, the current and voltage waves have the same signs, whereby the equation u=Z.sub.0 i is obtained.
U.S. Pat. No. 4,351,011 (Liberman) uses an alternative in which, instead of treating the voltage and current waves separately, the product of voltage and current is formed, i.e. the power or its integral, i.e. the energy. The direction to a fault can then be determined by the sign of the instantaneous power or energy change. For an internal fault or a fault lying ahead, a negative sign is then obtained on the power or energy change, and for an external fault or a fault lying behind, a positive sign is obtained.
In German patent application No. P 28 41 009.5-32, the different algebraical combinations of the voltage and current waves are utilized for obtaining directional detection.
In the specification of U.S. patent application Ser. No. 743,930, filed in the name of G. Nimmersjo on June 12, 1985, a method and a device are described for locating a fault point on a transmission line based on voltage waves moving from a measuring point towards a fault point and corresponding waves reflected from a fault point, which waves are included in a travelling wave model of the line. The invention comprises carrying out, at certain regular time intervals, a measurement of the instantaneous values of the current and the voltage at the end point of the line, for example at a station. With these values and with the aid of the travelling wave model, the voltage at a number of control points along the line can be calculated. It is the travelling wave model described in patent application Ser. No. 743,930 that forms the basis of the directional detection according to the present invention.